High purity aluminum conductor used at ultra low temperature

ABSTRACT

The aluminum conductor having increase of its electric resistivity kept small at ultra low temperature of 30° K. or lower even after cyclic strain is given at ultra low temperature, by controlling the crystal structure of the high purity aluminum conductor with purity of 99.9-99.9999 wt %. The crystal structure consist of (i) a veritable single or a substantially single crystal consisting of a bundle of sub-grains which have their crystal axes in the same direction or in the directions within a couple of degrees of deviation as a whole which has a specific crystal axis of &lt;111&gt; or &lt;100&gt; or the crystal axes close thereto in the longitudinal direction of the aluminum conductor, or (ii) a polycrystal most of which grains have respective specific crystal axes of &lt;111&gt; and/or &lt;100&gt;, or the crystal axes close thereto with respect to each grain in the longitudinal direction of the aluminum conductor, and have specific grain size of 0.01 mm to 3.0 mm.

FIELD OF THE INVENTION

This invention relates to a high purity aluminum conductor used at ultralow temperature of 30° K. or lower and its production process, thealuminum conductor being used under those conditions where cyclic strainis given at ultra low temperature.

TECHNICAL BACKGROUND AND PRIOR ART

In those facilities and equipment which utilize a superconductor, aconductor, generally called a cryostatic stabilizer, is provided on andaround the superconductor to protect the superconductor by by-passingthe electric current to the conductor when the state ofsuperconductivity returns partly or completely to the state of normalconductivity due to external thermal, electric or magnetic disturbance.The transition from the state of superconductivity to the state ofnormal conductivity (usually called as "QUENCHING") is accompanied by I²R heat generation (wherein I means electric current and R means electricresistivity of the conductor) in the normal regions of the conductorwhere current flows.

High purity aluminum, because its electric resistivity is remarkably lowat ultra low temperature and in magnetic field, has been discussed forpossible use as such a cryostatic stabilizer. [F. R. Fickett,"Magneto-resistivity of Very Pure Polycrystalline Aluminum", Phy. Rev.B. Vol. 3, No. 6, 1971, p1941. "Superconducting Magnetic Energy Storage"Vol. 1: Basic R&D 1984-85, EPRI GS-7053, published by the Electric PowerResearch Institute in November 1990.]

The use of a cryostatic stabilizer made of high purity aluminum isplanned for Superconducting Magnetic Energy Storage (SMES) devices. Butin such facilities which store large quantities of electric power, hoopstresses are caused by the flow of current through the magnet, and whenelectric charging and discharging are repeated, cyclic tensile stressand compressive stress are given repeatedly to the superconductor andthe cryostatic stabilizer.

It is known that such cyclic stress which includes a plastic straincomponent at ultra low temperature gives an adverse influence on highpurity aluminum at ultra low temperature in the form of an increase inelectric resistivity. [Advances in Cryogenic Engineering. 22, 486-489(1976).]

Therefore, for those applications in which cyclic strain is given atultra low temperature to the cryostatic stabilizer of high purityaluminum, the high purity aluminum conductor component ought to be of arelatively larger cross section in view of a possible increase inelectric resistivity of the cryostatic stabilizer when in use, or theconductor should be so designed as to reduce plastic strain of thecryostatic stabilizer under the same stress by increasing the designstrength of the structural materials of SMES.

However, the above countermeasures require a large amount of materialswhen adopted for such large structures as utility scale SMES and aretherefore very costly.

Further, it is known in the report of the International Conference onCryogenic Materials, Applications and Properties, Shenyang, People'sRepublic of China, Jun. 7-10, 1988 that in the case of a high purityaluminum conductor with the same purity as that of the high purityaluminum in the present invention used at ultra low temperature, itselectric resistivity under cyclic strain does not remain low enough forthe stabilizer if the cyclic strain range is too high.

SUMMARY DESCRIPTION OF THE INVENTION

An object of the present invention resides in providing a high purityaluminum conductor used at ultra low temperature with purity of 99.9 to99.9999 weight %, preferably 99.99 to 99.9999 weight %, by whichincrease of its electric resistivity kept small under those conditionswhere cyclic strain is given at ultra low temperature. Herein, the wordsof ultra low temperature mean a temperature range of 30° K. or lower.

Another object of the present invention resides in providing aproduction process for the aluminum conductor.

The present inventors made thorough studies about the development of thehigh purity aluminum conductor by which increase in electric resistivityat ultra low temperature is kept small even after cyclic strain is givenat ultra low temperature. As a result, the inventors have found that bycontrolling the crystal structure of the high purity aluminum conductorso as to consist of (i) a single crystal which has a specific crystalaxis of <111> or <100>, or within an angle range of not greater than 10°in relation to the <111> or <100> axis in the longitudinal direction ofthe aluminum conductor, or (ii) a polycrystal most of which crystalgrains have respective specific crystal axes, namely the <111> and/or<100> axes and/or the axes within an angle range of not greater than 10°in relation to the <111> or <100> axis, in the longitudinal direction ofthe aluminum conductor, and have specific crystal grain size, namely themean crystal grain size is between 0.01 mm and 3.0 mm, the increase ofelectric resistivity of the aluminum conductor can be kept small atultra low temperature even after longitudinal cyclic strain is given atultra low temperature.

DETAILED DESCRIPTION OF THE INVENTION

The invention is applied to high purity aluminum with purity of 99.9 to99.9999 weight %, preferably 99.99 to 99.9999 weight %. In theinvention, the purity of high purity aluminum means weight percentobtained by deducting, from 100, the amount of metallic andsemi-metallic elements contained in the aluminum other than aluminumelement which are detected by, for example, GDMSS (Glow Discharge MassSpectroscopy). Such gas components contained in the aluminum as oxygen,hydrogen or chlorine atoms are not deducted.

High purity aluminum with purity level of less than 99.9 weight % isinadequate as a cryostatic stabilizer for facilities such as SMES usingsuperconductors, because, even at ultra low temperature, its electricresistivity is not low enough for the stabilizer.

A veritable single crystal can be produced by the method of strainanneal crystal growth. The Bridgemen method, Chalmers method orCzochralski method, which uses an oriented seed crystal, can be adoptedfor obtaining an aluminum conductor consisting of a substantially singlecrystal which has the <111> or <100> axis, or the crystal axes within anangle range of not greater than 10° in relation to the <111> or <100>axis in the longitudinal direction of the aluminum conductor. A very lowspeed particular continuous casting method can be adopted for obtainingan aluminum conductor consisting of the substantially single crystal.

A bundle type polycrystal made of a bundle of grains each of which israther columnar and is almost as long as the length of the aluminumconductor in the longitudinal directions of the conductor has the sameeffects as those of the substantially single crystal, and has specificand effective orientation mentioned above. And the same low speedpaticular continuous casting method can be adopted for obtaining such abundle of polycrystal.

A polycrystal, consisting of very coarse crystal grains, each of which(i) is as long as the diameter of the aluminum conductor in the lateraldirection of the conductor and (ii) is lined one after another in thelongitudinal direction of the conductor, and most of which grains havethe <111> and/or <100> axis and/or the crystal axes within an anglerange of not greater than 10° in relation to the <111> or <100> axis,respectively, in the longitudinal direction of the conductor, can workin a manner similar to the single crystal mentioned above as a highpurity aluminum conductor for use at ultra low temperature in thisinvention.

Further, an aluminum conductor consisting of a polycrystal, most ofwhich crystal grains have the <111> and/or <100> axis and/or the crystalaxes within an angle range of not greater than 10° in relation to the<111> or <100> axis, respectively, in the longitudinal direction of thealuminum conductor and have mean grain size between 0.01 mm and 3.0 mm,preferably 0.01 to 2.0 mm, also works effectively as a high purityaluminum used at ultra low temperature in this invention. An extrusioncrystal texture or re-crystallized texture is suitable for such apolycrystal. In this invention, such extrusion crystal texture orre-crystallized texture can be obtained by extrusion working of highpurity aluminum at 150° C. to 350° C. and in area reduction ratio of1/10 to 1/150, preferably 1/20 to 1/100, optionally cooling it to roomtemperature, subsequently heating it up to a temperature range of from250° C. to 530° C. and holding it at the temperature for 10 min. to 120min.

As will be apparent from Tables 1 and 2 shown herein, after cyclicstrain is given 3,000 times at ultra low temperature, the high purityaluminum of the invention has lower electric resistivity in liquidhelium than the aluminum used in the comparative examples and,therefore, has excellent properties as a cryostatic stabilizer used atultra low temperature.

EXAMPLE 1:

A high purity alminium rod with a purity of 99.999 weight % was extrudedat 280° C. by a hot extrusion press (1,500 ton extrusion press of NIHONTEKKO) from an initial diameter of 155 mm to 25 mm. After the rod wascut to obtain a 210 mm long sample, the sample was heated rapidly to450° C. and held at the temperature for 10 minutes. A 10 mm long piecewas cut out from one end section of the 210 mm long sample, and the cutsurface was machined and polished. After the polished surface layer ofthe piece was eliminated by chemical etching and it was confirmed thatthe sample consisted of a grain-form crystal, the <111> pole figure wasmeasured by x-ray diffraction (Schulz Reflection Method). As a result,<100> structure was found in that pole figure, and it was confirmed thatthe mean grain size was 0.4 mm by area mean method of etched surface.The residual 200 mm of the 210 mm long sample was lathed into a samplerod with a diameter of 10 mm and a length of 200 mm. The sample rod wasannealed at 250° C. for 2 hours and residual stress on the surface ofthe sample rod caused by lathing was removed. Further, the sample wassoaked with jigs into liquid helium, and its electric resistivity wasmeasured at 4.2° K. of ultra low temperature by the eddy current decaymethod.

In addition, the above 200 mm sample was given with 0.1% cyclic tensilestrain and compressive strain by 3,000 times with the temperature keptat 4.2° K. and, thereafter its electric resistivity was measured in theliquid helium by the eddy current decay method in the same manner asmentioned in the just above paragraph. The results are shown in Table 1.

EXAMPLE 2:

The same procedures as shown in EXAMPLE 1 were taken except that thepurity was 99.9995 weight % instead of 99.999 weight % and therecrystallization heat treatment at 300° C. for 60 minutes instead of at450° C. for 10 minutes. The results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Measurements on Polycrystalline Specimens                                                         Electric resistivity                                                Heat-  Mean              After 3000                                           treatment                                                                            grain    Before   cycles of                                            (°C.)                                                                         size     strain   strain                                               (minute)                                                                             (mm)     (nΩcm)                                                                           (nΩcm)                               ______________________________________                                        INVENTION   450      0.4      0.6    4.3                                      EXAMPLE 1    10                                                               INVENTION   300      0.1      0.6    3.3                                      EXAMPLE 2    60                                                               COMPARATIVE 300      0.4      0.58   9.70                                     EXAMPLE      60                                                               ______________________________________                                         *This data is derived from the report of International Conference on          Cryogenic Materials, Applications and Properties, Shenyang, People's          Republic of China, June 7-10, 1988 (Table 3).                            

EXAMPLE 3:

A substantially single crystal rod of high purity aluminum with purityof 99.999 weight % with a diameter of 20 mm and a length of 250 mm wasproduced by the Chalmers method so as to have its crystal axis <100> inthe longitudinal direction of the rod.

The rod was cut and lathed into a sample rod with a diameter of 10 mmand a length of 200 mm. The sample rod was annealed at 200° C. for 2hours and residual stress on the surface of the sample rod caused bycutting and lathing was removed.

By etching the single crystal sample rod, it was confirmed that thisheat treatment did not cause new re-crystallized grains to emerge on thesurface of the rod.

The sample rod was soaked with jigs into liquid helium and its electricresistivity was measured at 4.2° K. of ultra low temperature by the eddycurrent decay method.

In addition, the sample rod was given with 0.1% tensile strain andcompressive strain to electric resistivity saturation (500 cycles ofcyclic strain) with the temperature kept at 4.2° K., thereafter itselectric resistivity was measured in the liquid helium in the samemanner as mentioned in the just above paragraph. The results are shownin Table 2.

EXAMPLE 4:

The same procedures as shown in EXAMPLE 3 were taken except that thecrystal axis <111>, instead of <100>, was oriented to the longitudinaldirection of the rod. The results are shown in Table 2.

EXAMPLE 5:

The same procedures as shown in EXAMPLE 3 were taken except that thecrystal axis deviating from <100> by 6°, instead of <100>, was orientedto the longitudinal direction of the rod. The results are shown in Table2.

EXAMPLE 6:

The same procedures as shown in EXAMPLE 3 were taken except that thecrystal axis deviating from <111> by 6°, instead of <100>, was orientedto the longitudinal direction of the rod. The results are shown in Table2.

COMPARATIVE EXAMPLE 2:

The same procedures as shown in EXAMPLE 3 were taken except that thecrystal axis <110>, instead of <100>, and that 3,000 cycles of cyclicstrains are given, instead of 500 cycles, was oriented to thelongitudinal direction of the rod. The results are shown in Table 2.

COMPARATIVE EXAMPLE 3:

The same procedures as shown in EXAMPLE 3 were taken except that thecrystal axis deviating from <110> by 6°, instead of <100>, and that3,000 cycles of cyclic strains are given, instead of 500 cycles, wasoriented to the longitudinal direction of the rod. The results are shownin Table 2.

COMPARATIVE EXAMPLE 4:

The same procedures as shown in EXAMPLE 3 were taken except that thecrystal axis deviating from <100> by 15°, instead of <100>, and that3,000 cycles of cyclic strains are given, instead of 500 cycles, wasoriented to the longitudinal direction of the rod. The results are shownin Table 2.

COMPARATIVE EXAMPLE 5:

The same procedures as shown in EXAMPLE 3 were taken except that thecrystal axis deviating from <111> by 15°, instead of <100>, and that3,000 cycles of cyclic strains are given, instead of 500 cycles, wasoriented to the longitudinal direction of the rod. The results are shownin Table 2.

                  TABLE 2                                                         ______________________________________                                                           Electric resistivity                                                 Conductor                                                                              (4.2° K.)                                                     crystal  Before strain                                                                            At saturation                                             orientation                                                                            (nΩcm)                                                                             (nΩcm)                                    ______________________________________                                        INVENTION                                                                     EXAMPLE                                                                       3           100        0.14       1.90                                        4           111        0.13       1.68                                        5           deviating  0.14       5.11                                                    from 100                                                                      by 6°                                                      6           deviating  0.14       5.81                                                    from 111                                                                      by 6°                                                      COMPARATIVE                                                                   EXAMPLE                                                                       2           110        0.16       9.47                                        3           deviating  0.17       13.70                                                   from 110                                                                      by 6°                                                      4           deviating  0.14       9.69                                                    from 100                                                                      by 15°                                                     5           deviating  0.19       11.30                                                   from 111                                                                      by 15°                                                     ______________________________________                                    

What is claimed is:
 1. A high purity aluminum conductor of a cryostaticstabilizer used at an ultra low temperature of 30° K. or lower which isprovided on and around a superconductor, consisting essentially of 99.99to 99.9999 weight percent aluminum, wherein the aluminum conductor has apolycrystal structure consisting of crystal grains wherein most of saidgrains have a specific crystal axis of <100>, <111> or both of <100> and<111>, each of which is essentially oriented to the longitudinaldirection of the aluminum conductor.
 2. A high purity aluminum conductoraccording to claim 1, wherein most of the crystal grains have a specificcrystal axis of <100> or <111> or an axis within an angle range of notgreater than 10° in relation to any one <100> and <111> axis,respectively, in the longitudinal direction of the aluminum conductor.3. A high purity aluminum conductor according to claim 2, wherein mostof the crystal grains have the mean grain size of between 0.01 mm and3.0 mm.
 4. A high purity aluminum conductor according to claim 1,wherein most of the crystal grains have a specific crystal axis of <100>in the longitudinal direction of the aluminum conductor.
 5. A highpurity aluminum conductor according to claim 1, wherein most of thecrystal grains have a specific crystal axis of <111> in the longitudinaldirection of the aluminum conductor.
 6. A high purity aluminum conductoraccording to claim 1, wherein most of the crystal grains have a specificcrystal axis of both of <100> and <111> in the longitudinal direction ofthe aluminum conductor.
 7. A high purity aluminum conductor of acryostatic stabilizer used at an ultra low temperature of 30° K. orlower which is provided on and around a superconductor, said high purityaluminum conductor consisting essentially of 99.995 to 99.9999 weightpercent aluminum, wherein the aluminum conductor has a polycrystalstructure consisting of crystal grains wherein most of said grains havea specific crystal axis of <100>, <111> or an axis within an angle rangeof not greater than 10° in relation to any one <100> and <111> axis,respectively, in the longitudinal direction of the aluminum conductor,and wherein the mean crystal grain size is 0.01 mm to 2.0 mm.
 8. A highpurity aluminum conductor of a cryostatic stabilizer used at an ultralow temperature of 30° K. or lower which is provided on and around asuperconductor, consisting essentially of 99.99 to 99.9999 weightpercent aluminum, wherein the aluminum conductor has a single crystalstructure having a specific crystal axis of <100> or <111> each of whichis essentially orientated to the longitudinal direction of the aluminumconductor.
 9. A high purity aluminum conductor of a cryostaticstabilizer used at an ultra low temperature of 30° K. or lower which isprovided on and around a superconductor, said high purity aluminumconductor consisting essentially of 99.99 to 99.9999 weight percentaluminum, wherein said high purity aluminum conductor consists of averitable single crystal or a substantially single crystal, wherein thecrystal has a specific crystal axis of <100>, <111> or an axis within anangle range of not greater than 10° in relation to any one of <100> and<111> axes, respectively, in the longitudinal direction of the aluminumconductor.